A young person can progress to an apprenticeship at 16 or 18, at a level to suit their needs and experience. Young people can go on to do an apprenticeship after GCSEs, A Levels, BTEC Nationals or another apprenticeship. If they want to do an apprenticeship after a degree they will have to get funding from an employer or help fund it themselves.
After A Levels or a Level 3 vocational qualification, a young person can apply for either an Advanced Apprenticeship (Level 3) or a Higher Apprenticeship (Level 4 and above). If they do not have relevant previous experience in the job sector, even though they already have a Level 3 qualification, they might need to do an Advanced Apprenticeship (Level 3) to build up the knowledge and skills required for the job.
For example, if a young person with A Levels or a Level 3 BTEC has relevant previous experience, for example they have done a Business Studies course and they would like to apply for an apprenticeship in Office Management, then their previous experience could contribute to the apprenticeship, so they might be considered for a Higher Apprenticeship (Level 4 and above). These are quite competitive and there is a limited supply. However, if a young person wants to do an engineering apprenticeship, for example, and they have no previous experience of this job area then they are likely to have to start at the Advanced Apprenticeship (Level 3) level to gain the specific job sector skills.
See the question 'What is an apprenticeship?' for more information about what's involved.
Careerpilot has a whole section about apprenticeships which explain what they are, the levels, pay, how to find one, etc. You can see the apprenticeship section here.